A typical known co-axial line to waveguide junction is shown in section in FIG. 1 of the accompanying drawings.
Referring to FIG. 1, a length of waveguide 1 has flanges 2 and 3 at each end. Flange 2 provides the means whereby a TR cell and an associated receiver may be connected, whilst flange 3 provides the means whereby a source of generated r.f. energy (e.g. a magnetron) to be transmitted may be connected. Along the length of the waveguide section 1, a further section of waveguide 4 is joined. Waveguide section 4 has, at its end remote from the waveguide section 1, a flange 5, by means of which a cavity 6 is connected. Introduced into cavity 6 is the inner conductor 7 of a co-axial line connector 8. Co-axial line connector 8 is provided to receive the co-axial transmission line leading to a common transmitter/receiver aerial.
The distance A between the centre of the waveguide section 4 and the effective short circuit provided by a TR cell connected to flange 2 is such that when the TR cell is switched on by r.f. energy originating from the r.f. source connected to flange 3, a short circuit is transformed to the junction of the waveguide sections 4 and 1, such that effectively a waveguide bend is produced, so that r.f. energy is coupled from the r.f. source connected to flange 3 to the cavity 6 and thus to the co-axial line connector 8.
The distance B from the centre of waveguide section 4 to the flange 3 is chosen such that when the r.f. source is inoperative, a short circuit is transformed to the junction of the waveguide sections 4 and 1, such that a waveguide bend is effectively provided connecting the co-axial line connector 8, via the cavity 6 to the receiver connected to flange 2.
Not only is the co-axial line to waveguide junction described above undesirably large for many purposes, but also the extensive cavities within the junction tend to sustain parasitic modes.